I am cheap and lazy so I want to get maximum use from an 8 by 4 sheet of plastic without having to cut it into a thousand petals and still have it work with the rest of the design.
What I came up with will probably work for me.
But I wanted to share the method so that it can work for you, and perhaps for disaster relief too.
Step 1: Design of the parabolic curve
However the method is valid.
I will leave the image notes to explain things for the moment.
I start with the mirror straight across from the target ball because if you start anywhere else, it is hard to figure out how big your dish will be.
There are 2 ways to do the mirror angle.
The mirror is set up at half the angle of the light hitting the mirror and the light bouncing away from the mirror. The parallel lines method is much quicker than measuring angles but you need to check as errors creep in.
I never learned cad.
I am sure this will be perfected on computer aided design programs.